Gooseneck condenser microphones (hereinafter, referred sometimes to as a “condenser microphone”) are used favorably in conference halls for, for example, international conferences, TV studios, and the like because the appearance thereof is simple, and the angle and height thereof relative to a speaker can be adjusted easily.
The gooseneck condenser microphones come substantially in two types: one is a separate type in which a microphone unit and an output module part are separated from each other (refer to FIG. 3), and the other is an integral type in which the microphone unit and the output module part are connected to each other (refer to FIG. 4).
A separate type condenser microphone A shown in FIG. 3 is configured so that a condenser microphone unit (hereinafter, referred sometimes to as a “microphone unit”) 10 and an output module part 20 including a sound signal output circuit board 21 for the microphone unit are separated from each other. The microphone unit 10 is supported on the front end side of a support pipe 30, and the output module part 20 is attached to the rear end (proximal end) side of the support pipe 30.
The support pipe 30 includes a flexible shaft 31. In this example, the flexible shaft 31 has a front end-side flexible shaft 31a and a rear end-side flexible shaft 31b, and a junction pipe 32 consisting of a metallic straight tube is interposed therebetween. The output module part 20 has a shield housing 20a, and is placed on a base such as a table via a fixing device (not shown).
The microphone unit 10 and the sound signal output circuit board 21 of the output module part 20 are connected electrically to each other via a microphone cable 40 inserted through the support pipe 30. As the microphone cable 40, a two-core shield covered cable is used. Also, in the separate type, an output connector 22 is incorporated in the output module part 20 together with the sound signal output circuit board 21.
Usually, as the output connector 22, an output connector having No. 1 pin for grounding, No. 2 pin on the hot side of a signal, and No. 3 pin on the cold side thereof, which are specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Audio Equipment”, is used.
Although not shown in FIG. 3, a field effect transistor (FET) serving as an impedance converter is mounted in the microphone unit 10. In the case of separate type, the microphone cable 40 is an unbalanced transmission line. On the microphone unit 10 side, one core wire of the microphone cable 40 is connected to the drain side of the FET as a power source line, the other core wire thereof is connected to the source side as a signal line, and a shield cover conductor is connected to a unit case on the ground side. The source of the FET is also connected to the unit case (ground).
On the other hand, on the output module part 20 side, the power source line and the signal line of the microphone cable 40 are connected to predetermined terminals of the sound signal output circuit board 21, and the shield cover conductor is connected to the ground (grounded circuit) of the sound signal output circuit board 21. The ground of the sound signal output circuit board 21 is connected to the No. 1 pin of the output connector 22, and the No. 1 pin is also connected to the shield housing 20a of the output module part 20. That is, the No. 1 pin is the base point of grounding.
The output connector 22 is connected to a phantom power source via a balanced two-core shielded cable (both not shown). The output module part 20 is sometimes called a power module part because it supplies power to the microphone unit 10.
In contrast, an integral type condenser microphone B shown in FIG. 4 includes a microphone body M configured so that the microphone unit 10 and the output module part 20 are connected to each other, and the microphone body M is supported by the front end side of the support pipe 30. To the rear end side of the support pipe 30, a proximal housing 50 having only the output connector 22 is attached.
In this condenser microphone B, the sound signal output circuit board 21 in the output module part 20 and the output connector 22 in the proximal housing 50 are connected electrically to each other via the microphone cable 40.
In the case of the integral type, the microphone cable 40 is a balanced transmission line. On the output module part 20 side, the hot-side signal line and the cold-side signal line of the microphone cable 40 are connected to the drain side and the source side of the FET, respectively, via a predetermined wiring of the sound signal output circuit board 21, and the shield cover conductor is connected to the ground of the sound signal output circuit board 21. The source of the FET and the ground of the sound signal output circuit board 21 are connected to the shield housing 20a on the ground side.
On the other hand, on the proximal housing 50 side, the hot-side signal line and the cold-side signal line of the microphone cable 40 are connected to the No. 2 pin and the No. 3 pin of the output connector 22, respectively, and the shield cover conductor is connected to the No. 1 pin. The No. 1 pin is also connected to the proximal housing 50, so that in this condenser microphone B as well, the No. 1 pin is the base point of grounding.
In this example, the support pipe 30 consists of the rear end-side flexible shaft 31b and the junction pipe 32. In the rear end portion of the shield housing 20a of the output module part 20, a metallic coupler (connecting member) 20b is provided, and the output module part 20 is connected to the junction pipe 32 via the coupler 20b. 
In both of the above-described separate type condenser microphone A and integral type condenser microphone B, the support pipe 30 and the shield covered cable of the microphone cable 40 function as antennas and are liable to pick up noise (disturbance electromagnetic waves) from the outside.
In addition, the flexible shaft 31 is produced by a coil spring consisting of a round wire rod made of steel or the like and a triangular wire rod made of a plastically deformable copper alloy or the like. The contact portions of these wire rods have impedance though having a low resistance value (for example, about 10Ω), so that in terms of high frequency, it cannot be said that the shield of the microphone cable 40 is complete.
Therefore, when a strong disturbance electromagnetic waves are applied to the microphone cable 40, the disturbance electromagnetic waves intrude into the microphone unit 10 and the output module part 20 as a high-frequency current, and are detected by a semiconductor device of the FET or the like, which results in the generation of noise.
In particular, from a cellular phone, considerably strong electromagnetic waves (for example, in the range of about several centimeters to several tens centimeters, field intensity reaching several ten thousands times the intensity of electric field generated in the city by commercial electric waves) are radiated, so that in the field of condenser microphone, an urgent need is to take measures against electromagnetic waves caused by the use of a cellular phone at close range.
Accordingly, concerning the separate type condenser microphone A shown in FIG. 3, the present applicant has proposed, in Japanese Patent Application Publication No. 2006-33216, a technique in which the skin (external sheath) of at least a portion where wiring is installed of the microphone cable inserted through the support pipe is removed to expose the shield cover conductor, and the shield cover conductor is brought into contact with the flexible shaft at multiple points, whereby the resistance value of flexible shaft is made a very small value to improve the shielding function against electromagnetic waves, and thereby the generation of noise caused by disturbance electromagnetic waves is restrained effectively.
The invention described in Japanese Patent Application Publication No. 2006-33216 is effective to some degree for the integral type condenser microphone B shown in FIG. 4 as well, but has a problem described below in the case of integral type.
In the integral type condenser microphone B, the ground (grounded circuit) of the sound signal output circuit board 21 in the output module part 20 and the ground base point (No. 1 pin) of the entire of microphone are separated from each other by the length of the support pipe 30, the support pipe 30 and the shield housing 20a of the output module part 20 are connected electrically to each other, and with the decrease in size of the microphone body M, which is a head unit, it becomes difficult to make the shield of portions of the housing complete. Therefore, the integral type condenser microphone B still has a problem of the generation of noise caused by disturbance electromagnetic waves.
Accordingly, an object of the present invention is to prevent noise from being generated in the integral type condenser microphone shown in FIG. 4 even if a cellular phone that radiates strong electromagnetic waves is used at close range.